The process for producing acrylonitrile or methacrylonitrile by contacting propylene, isobutene or tertiary butanol with a catalyst in a vapor phase in the presence of ammonia and molecular oxygen has been widely known as ammoxydation reaction of olefins and industrially practiced. Many multi-component oxide catalysts containing molybdenum, bismuth and iron, which are usable for this reaction, have been disclosed in JP-B-51-6649, JP-B-51-33888, JP-B-56-52013, JP-B-59-50667, JP-B-60-36812, etc.
These catalysts, however, have the problem that when they are used for fluidized bed ammoxydation reaction of propylene, isobutene or tertiary butanol, the yield of acrylonitrile or methacrylonitrile decreases with time. Many factors are conceivable as causes of such decrease of yield and they are yet to be elucidated in more detail, but it is considered that the loss of molybdenum from the catalyst in the course of ammoxydation reaction is one of the causes. Various methods such as mentioned below have been proposed for preventing drop of catalyst performance by supplementing the loss of molybdenum: (1) the catalyst reduced in performance is once taken out of the reaction system and impregnated with a solution containing molybdenum; (2) a molybdenum compound supported on a carrier is added to the reaction system; (3) a molybdenum compound not supported on a carrier is added to the reaction system; (4) an oxide catalyst containing molybdenum and other necessary metal elements, of high molybdenum content, is added.
Regarding the method (1), specifically, JP-B-55-49541 (E.I. du Pont de Nemours & Co.) and JP-B-5-33100 (Enichem Sintesi Spa) disclose the idea that a molybdenum-containing multi-component oxide catalyst deactivated after use for the ammoxydation reaction be impregnated with a solution containing molybdenum or a solution containing molybdenum and bismuth and then calcined to form a regenerated catalyst, and this regenerated catalyst be used again for the ammoxydation reaction.
Relating to the method (2), JP-B-58-57422 (U.S. Pat. No. 3,882,159: The Standard Oil Company) mentions addition of silica-supported molybdenum oxide to the reactor during the fluidized bed ammoxydation reaction using a molybdenum-containing multi-component oxide catalyst. JP-A-59-193136 (Ube Industries, Ltd.) proposes to carry out ammoxydation reaction in a fixed bed in the presence of a molybdenum-containing oxide catalyst and molybdenum oxide supported on an inert carrier.
Concerning the method (3), DE-A-3,311,521 (SKW) discloses addition of a non-supported molybdenum compound, preferably molybdenum trioxide or ammonium molybdate, to the catalyst to be regenerated in a ratio of 0.25-2.5% by weight during the fluidized bed ammoxydation reaction using a molybdenum-containing oxide catalyst. Also, JP-B-2-56938 (Nitto Chemical Industries Co., Ltd.) proposes addition of non-supported solid molybdenum during the fluidized bed ammoxydation reaction using an oxide catalyst containing at least one element selected from the group consisting of vanadium, molybdenum and tungsten in addition to iron, antimony and tellurium.
According to the method (1), however, since the catalyst reduced in performance needs to be taken out of the reaction system and regenerated, the process is complicated, resulting in a substantial economical loss. So, means for recovering the catalyst performance in the course of the reaction have been sought, and this prompted proposal of the methods (2) and (3) in which a supported or non-supported molybdenum compound is added to the reaction system. These methods, however, still involve the problem that since the molybdenum compound to be added is different in physical properties from the catalyst in the reaction system, the molybdenum compound added in large quantities tends to adhere and accumulate on the cooling coils in the reactor or on the heat exchanger at the exit of the reactor, and such deposits need to be removed. Also, as mentioned in JP-B-2-56938, deactivation of the iron/antimony-based oxide catalyst is not considered to be caused by the loss of molybdenum, but it is considered that the recovery of catalyst activity by the addition of molybdenum is credited to the subsequent creation of new active sites on the catalyst. Therefore, the necessity of continuous addition of molybdenum in the catalyst system reflects the fact that the composition of the iron/antimony-based oxide catalyst changes with time, and thus it is considered that there is a limit to the long-time maintenance of catalyst performance. Further, in case a supported molybdenum compound is added to the reaction system, there arises the problem that the remaining carrier is accumulated in the reaction system.
Recently, with the background of these facts, there has been proposed the method (4) according to which the problems of the methods (1) to (3) are resolved by increasing the molybdenum content of the catalyst and also adding a compound whose elemental composition excluding molybdenum is analogous to that of the catalyst in the reaction system. For instance, U.S. Pat. No. 5,177,048 (China Petro-Chemical Corp.) and U.S. Pat. No. 5,378,668 (EC Erdolchemie GmbH) propose addition of an oxide catalyst containing molybdenum and other metal elements, of high molybdenum content, during the fluidized bed ammoxydation reaction using a molybdenum-containing multi-component oxide catalyst. It was found, however, that according to this method, since the catalyst contains metal elements other than molybdenum, there arise the problems such as difficulties in maintaining the composition of the catalyst in the reaction system and excess amounts of the additives, which makes it unable to realize stabilized long-time continuous production of acrylonitrile or methacrylonitrile.
An object of the present invention is to provide a process for producing acrylonitrile or methacrylonitrile by carrying out a fluidized bed ammoxydation reaction using a multi-component oxide catalyst containing molybdenum, which process is capable of preventing the yield from dropping with time and producing the objective product stably over a long time.